Expose compaction summary/prompt to users (keep thread/compacted or add fields to contextCompaction)

[ignatremizov]

When a thread auto-compacts (or when the user runs /compact), users lose visibility into what the model now actually has in-context. This makes it very hard to understand or debug behavior changes after compaction, and it erodes trust when the agent forgets/denies earlier work.

A concrete example is #5957, where auto-compaction caused the agent to forget it had edited files and it confidently denied having made those edits.

Current state

Recent work introduced contextCompaction as a v2 ThreadItem / item lifecycle marker (see the recent compaction work, e.g. #10034 and the item notification tests in #10123). However, contextCompaction currently only carries { id }.

At the same time, the existing thread/compacted notification (ContextCompactedNotification) can carry summary and the full compacted message (prompt), but it is now marked Deprecated: Use ContextCompaction item type instead.

Request

Please don't remove/deprecate the only user-visible carrier of compaction text until there's an equivalent replacement.

Two viable options:

1. Keep thread/compacted (ContextCompactedNotification) as a supported event (even if marked legacy), because it's extremely useful for users/UIs to show the compacted summary and/or compacted prompt.
2. Alternatively, extend the contextCompaction item to include summary?: string and message?: string (optional) so new clients can rely purely on the item stream. Then thread/compacted can be deprecated later with a clear timeline.

Why this matters

Showing the compacted text lets the user immediately see why the agent forgot or misunderstood something after compaction. It also helps debugging support requests, since a repro often hinges on whether compaction dropped key state.

Implementation note: for providers/paths where plaintext isn't available (e.g. remote compaction that can return encrypted content), summary/message can simply be omitted; but when available, exposing them to the UI is very valuable.

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Ref:

"compaction" #10034

[ignatremizov]

@aibrahim-oai

[ignatremizov]

I implemented the TUI display for the compacted summary/prompt in my fork: commit ef2eeaea8 on branch feat/compact-summary-output. It shows the compacted text after compaction so users can see what the model retained.

This currently relies on the thread/compacted (ContextCompactedNotification) payload to get summary/message. If compacted is removed without an equivalent summary/message on contextCompaction, the TUI won’t be able to show compaction summaries at all.